Project Summary This multidisciplinary research project will test the novel hypothesis that Dll4-triggered signaling contributes to the pathogenesis of atherosclerosis. We will focus on macrophage-derived proteolytic activity, the key feature typical of arterial remodeling associated with the onset of acute coronary events. We previously reported that Dll4 (a ligand of Notch signaling) promotes expression or activation of pro-inflammatory factors (e.g., iNOS, NF-?B) in cultured macrophages. The role of Notch signaling is strictly cell-type- and context- dependent, and in vivo functions of the Notch pathway in macrophages remain unknown. Using mouse models, the present study will explore the role of the Dll4-Notch axis in activation of plaque macrophages and development of atherosclerosis. Specific Aim 1 will examine whether Dll4 antibody administration attenuates macrophage activation and atherogenesis in Ldlr-/- mice. We will also use macrophage-targeted in vivo delivery of Dll4 siRNA to determine the relative contribution of macrophage Dll4. Specific Aim 2 will address the role of Notch3 based on our data that suggested its pro-atherogenic role. In Notch3-transgenic and null mice to test the hypothesis that this Notch receptor promotes macrophage activation and athrogenesis. These complementary studies will offer novel mechanisms of macrophage activation and atherosclerosis, and will also provide proof of concept that the Dll4-Notch3 pathway can be a therapeutic target for atherosclerosis, its complications, and other vascular diseases.